fonts: fix CJK fallback font loading

clearing font charset restrictions by copying a font descriptor's attributes and
re-creating it sometimes doesn't work; for example, with 'PingFang SC Regular'
(derived from '.AppleSystemUIFontMonospaced' in `discoverCodepoint(...)`, using
`CTFontCreateForString(...)`), `face.hasCodepoint(cp, p_mode)` will erroneously return
false after copying, and the font will be rejected. this causes fallback font
lookup to fail for CJK characters when using '.AppleSystemUIFontMonospaced'.
(note: if you first render a non-CJK character that loads a font that contains
CJK characters, everything will work fine as the CJK fallback path isn't hit.)

I'm fixing this by only clearing charset restrictions if we know that we've applied
them to the font (using `CoreText.discover(...)`, not hitting the special CJK path),
but I'm not sure how fragile the charset restriction clearing is for other fonts.

Author: recursiveforte

src/font/discovery.zig

@@ -372,6 +372,7 @@ pub const CoreText = struct {
             .alloc = alloc,
             .list = zig_list,
             .variations = desc.variations,
+            .charset_restriction = true,
             .i = 0,
         };
     }
@@ -409,6 +410,7 @@ pub const CoreText = struct {
                 .alloc = alloc,
                 .list = list,
                 .variations = desc.variations,
+                .charset_restriction = false,
                 .i = 0,
             };
         }
@@ -433,6 +435,7 @@ pub const CoreText = struct {
                 .alloc = alloc,
                 .list = list,
                 .variations = desc.variations,
+                .charset_restriction = false,
                 .i = 0,
             };
         }
@@ -828,6 +831,8 @@ pub const CoreText = struct {
         alloc: Allocator,
         list: []const *macos.text.FontDescriptor,
         variations: []const Variation,
+        // We used a charset restriction to filter, and need to remove it
+        charset_restriction: bool,
         i: usize,
 
         pub fn deinit(self: *DiscoverIterator) void {
@@ -849,6 +854,10 @@ pub const CoreText = struct {
             const desc = desc: {
                 const original = self.list[self.i];
 
+                if (!self.charset_restriction) {
+                    break :desc original;
+                }
+
                 // For some reason simply copying the attributes and recreating
                 // the descriptor removes the charset restriction. This is tested.
                 const attrs = original.copyAttributes();